1. Field of the Invention
The present invention relates generally to liquid crystals, and, more particularly, to a substrate that prealigns a liquid crystal layer.
2. Description of Related Art
One type of liquid crystal electro-optical device is constructed by placing a thin layer of liquid crystal between two transparent plates that have each been coated with a layer of an electrical conductor on its interior face. When no electric field is applied (field-OFF) between the conductive layers, the director of the liquid crystal is in one state. (The "director" of the liquid crystal is the average macroscopic direction of the long molecular axis of the liquid crystal molecules.) When an electric field is applied (field-ON), the director reorients to another state. Because the liquid crystal is birefringent, the two states will have different refractive indices, and in many cases the thin layer of liquid crystal will be birefringent for light directed against the transparent plates. The state change responsive to an applied electric field is the basis for liquid crystal devices that control light, such as displays and liquid crystal light valves used in projectors.
In its usual form, a liquid crystal light valve is a device that modulates a polarized projection light beam directed against one side of the light valve according to a writing light beam directed against the other side. The polarized projection light beam enters the light valve through one of the transparent electrodes, usually termed the counterelectrode, passes through the liquid crystal layer, and is reflected from a mirror on the other electrode. The projection light beam passes back through the liquid crystal layer and the counterelectrode, and through an external analyzer. The polarizing and analyzing functions can often be accomplished by a single polarizing beam splitter through which the projection light beam passes before and after passing through the liquid crystal. The operation of this and other types of liquid crystal light valves is discussed in greater detail in numerous technical publications; see, for example, "Progress in Liquid Crystal Light Valves", by W. P. Bleha, in Laser Focus/Electro Optics, October 1983, pages 111-120.
In this type of liquid crystal light valve, an electric bias field is applied through the liquid crystal layer by a voltage on the conductive electrodes. The liquid crystal is oriented responsive to this electric bias field. The director of the liquid crystal is initially, in the electric field OFF state, aligned perpendicular to the plates. Application of the electric field causes the director to rotate toward the plane of the plate, changing the light transmittance of the device. The writing light pattern modulates the electric field, changing the phase retardation of the light passing through the liquid crystal, which in turn modulates the projection light beam passing through the analyzer.
For certain applications it is desirable to orient the directors of these liquid crystals such that they assume parallel alignment with respect to the surface of a substrate. Such an alignment is necessary for certain television projection displays and color symbology light valves. In general, parallel alignment is desirable if an electric field is applied across the liquid crystal normal to the surfaces, and if the liquid crystal has a positive dielectric anisotropy, so that when the molecules lie parallel to the substrate surface they can be tilted towards a perpendicular orientation.
U.S. Pat. No. 4,030,997, issued Jun. 21, 1977, to L. J. Miller et al, U.S. Pat. No. 4,022,934, issued May 10, 1977 to L. J. Miller, and U.S. Pat. No. 5,011,267, issued Apr. 30, 1991, to L. J. Miller et al describe methods for aligning liquid crystal molecules in the homeotropic (or perpendicular) or the tilted perpendicular orientation. While these patents are certainly suitable for the purposes intended, the homeotropic liquid crystal alignment processes described are useful with liquid crystals having a negative dielectric anisotropy.
There are also methods for aligning the liquid crystal molecules parallel to the surface ("homogeneous" alignment). Such parallel liquid crystal alignment processes are useful with liquid crystals having a positive dielectric anisotropy.
The simplest alignment method is to rub the surface, but this produces a streaky surface alignment which is not very uniform on a microscopic scale. Another method is to coat the surface with SiO.sub.2 and etch the surface with an ion beam, as described in U.S. Pat. No. 4,153,529 by M. J. Little et al. This produces a parallel alignment with "splay," in which the alignment direction varies systematically across the substrate, due to divergence of the ion beam. Yet another method is oblique deposition as reported by J. L. Janning, Applied Physics Letters, Vol. 21, pp.173-174 (1972), but this also produces "splay" in the alignment because of divergence in the deposition beam.
The use of beams, such as ion beams or evaporated beams of SiO.sub.x molecules or particles, are inherently divergent, and the alignment of the liquid crystal on the surface replicates the divergence of the beam. For example, the molecules at one point on a surface may be aligned in a direction that differs by 3.degree. to 7.degree. from the direction of alignment an inch away on the same surface. This non-uniformity can be seen when the device is viewed between crossed polarizers.
There remains a need for a process which aligns nematic liquid crystals in a substantially parallel, homogeneous alignment. The present invention fulfills this need, and further provides related advantages.